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1. From Carbon to Cobalt: Chemical Compositions and Ages of z ∼ 0.7 Quiescent Galaxies

   https://doi.org/10.3847/1538-4357/acc176  

   Beverage, Aliza G.; Kriek, Mariska; Conroy, Charlie; Sandford, Nathan R.; Bezanson, Rachel; Franx, Marijn; van der Wel, Arjen; Weisz, Daniel R. (May 2023, The Astrophysical Journal)

   Abstract We present elemental abundance patterns (C, N, Mg, Si, Ca, Ti, V, Cr, Fe, Co, and Ni) for a population of 135 massive quiescent galaxies atz∼ 0.7 with ultra-deep rest-frame optical spectroscopy drawn from the LEGA-C survey. We derive average ages and elemental abundances in four bins of stellar velocity dispersion (σ_v) ranging from 150–250 km s⁻¹using a full-spectrum hierarchical Bayesian model. The resulting elemental abundance measurements are precise to 0.05 dex. The majority of elements, as well as the total metallicity and stellar age, show a positive correlation withσ_v. Thus, the highest dispersion galaxies formed the earliest and are the most metal-rich. We find only mild or nonsignificant trends between [X/Fe] andσ_v, suggesting that the average star formation timescale does not strongly depend on velocity dispersion. To first order, the abundance patterns of thez∼ 0.7 quiescent galaxies are strikingly similar to those atz∼ 0. However, at the lowest-velocity dispersions, thez∼ 0.7 galaxies have slightly enhanced N, Mg, Ti, and Ni abundance ratios and earlier formation redshifts than theirz∼ 0 counterparts. Thus, while the higher-mass quiescent galaxy population shows little evolution, the low-mass quiescent galaxies population has grown significantly over the past 6 Gyr. Finally, the abundance patterns of bothz∼ 0 andz∼ 0.7 quiescent galaxies differ considerably from theoretical prediction based on a chemical evolution model, indicating that our understanding of the enrichment histories of these galaxies is still very limited. 

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2. The Heavy Metal Survey: The Evolution of Stellar Metallicities, Abundance Ratios, and Ages of Massive Quiescent Galaxies since z ∼ 2

   https://doi.org/10.3847/1538-4357/ad372d  

   Beverage, Aliza G.; Kriek, Mariska; Suess, Katherine A.; Conroy, Charlie; Price, Sedona H.; Barro, Guillermo; Bezanson, Rachel; Franx, Marijn; Lorenz, Brian; Ma, Yilun; et al (May 2024, The Astrophysical Journal)

   Abstract We present the elemental abundances and ages of 19 massive quiescent galaxies atz∼ 1.4 andz∼ 2.1 from the Keck Heavy Metal Survey. The ultradeep LRIS and MOSFIRE spectra were modeled using a full-spectrum stellar population fitting code with variable abundance patterns. The galaxies have iron abundances between [Fe/H] = −0.5 and −0.1 dex, with typical values of −0.2 [−0.3] atz∼ 1.4 [z∼ 2.1]. We also find a tentative $\log {\sigma }_v$–[Fe/H] relation atz∼ 1.4. The magnesium-to-iron ratios span [Mg/Fe] = 0.1–0.6 dex, with typical values of 0.3 [0.5] dex atz∼ 1.4 [z∼ 2.1]. The ages imply formation redshifts ofz_form= 2–8. Compared to quiescent galaxies at lower redshifts, we find that [Fe/H] was ∼0.2 dex lower atz= 1.4–2.1. We find no evolution in [Mg/Fe] out toz∼ 1.4, though thez∼ 2.1 galaxies are 0.2 dex enhanced compared toz= 0–0.7. A comparison of these results to a chemical evolution model indicates that galaxies at higher redshift form at progressively earlier epochs and over shorter star formation timescales, with thez∼ 2.1 galaxies forming the bulk of their stars over 150 Myr atz_form∼ 4. This evolution cannot be solely attributed to an increased number of quiescent galaxies at later times; several Heavy Metal galaxies have extreme chemical properties not found in massive galaxies atz∼ 0.0–0.7. Thus, the chemical properties of individual galaxies must evolve over time. Minor mergers also cannot fully account for this evolution as they cannot increase [Fe/H], particularly in galaxy centers. Consequently, the buildup of massive quiescent galaxies sincez∼ 2.1 may require further mechanisms, such as major mergers and/or central star formation. 

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3. Stellar Metallicities and Elemental Abundance Ratios of z~1.4 Massive Quiescent Galaxies

   https://doi.org/10.3847/2041-8213/ab2e75  

   Kriek, Mariska; Price, Sedona; Conroy, Charlie; Suess, Katherine; Mowla, Lamiya; Pasha, Imad; Bezanson, Rachel; van Dokkum, Pieter; Barro, Guillermo (August 2019, The astrophysical journal)

   The chemical composition of galaxies has been measured out to z∼4. However, nearly all studies beyond z∼0.7 are based on strong-line emission from HII regions within star-forming galaxies. Measuring the chemical composition of distant quiescent galaxies is extremely challenging, as the required stellar absorption features are faint and shifted to near-infrared wavelengths. Here, we present ultradeep rest-frame optical spectra of five massive quiescent galaxies at z∼1.4, all of which show numerous stellar absorption lines. We derive the abundance ratios [Mg/Fe] and [Fe/H] for three out of five galaxies; the remaining two galaxies have too young luminosity-weighted ages to yield robust measurements. Similar to lower-redshift findings, [Mg/Fe] appears positively correlated with stellar mass, while [Fe/H] is approximately constant with mass. These results may imply that the stellar mass–metallicity relation was already in place at z∼1.4. While the [Mg/Fe]−mass relation at z∼1.4 is consistent with the z<0.7 relation, [Fe/H] at z∼1.4 is ∼0.2 dex lower than at z<0.7. With a [Mg/Fe] of 0.44+0.08 the most -0.07 massive galaxy may be more α-enhanced than similar-mass galaxies at lower redshift, but the offset is less significant than the [Mg/Fe] of 0.6 previously found for a massive galaxy at z=2.1. Nonetheless, these results combined may suggest that [Mg/Fe] in the most massive galaxies decreases over time, possibly by accreting low- mass, less α-enhanced galaxies. A larger galaxy sample is needed to confirm this scenario. Finally, the abundance ratios indicate short star formation timescales of 0.2–1.0 Gyr. 

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4. Predictions for complex distributions of stellar elemental abundances in low-mass galaxies

   https://doi.org/10.1093/mnras/stac834  

   Patel, Preet B.; Loebman, Sarah R.; Wetzel, Andrew; Faucher-Giguère, Claude-André; El-Badry, Kareem; Bailin, Jeremy (March 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We investigate stellar elemental abundance patterns at $$z$$ = 0 in eight low-mass ($$M_{*}=10^{6}{-}10^{9}\ \text{M}_{\odot }$$) galaxies in the Feedback in Realistic Environments cosmological simulations. Using magnesium (Mg) as a representative α-element, we explore stellar abundance patterns in magnesium-to-iron ([Mg/Fe]) versus iron-to-hydrogen ([Fe/H]), which follow an overall monotonic trend that evolved slowly over time. Additionally, we explore three notable secondary features in enrichment (in three different case-study galaxies) that arise from a galaxy merger or bursty star formation. First, we observe a secondary track with a lower [Mg/Fe] than the main trend. At $$z$$ = 0, stars from this track are predominantly found within 2–6 kpc of the centre; they were accreted in a 1:3 total-mass-ratio merger ∼0.4 Gyr ago. Second, we find a distinct elemental bimodality that forms following a strong burst in star formation in a galaxy at $$t_{\text{lookback}}\, \sim 10$$ Gyr. This burst quenched star formation for ∼0.66 Gyr, allowing Type Ia supernovae to enrich the system with iron (Fe) before star formation resumed. Third, we examine stripes in enrichment that run roughly orthogonal to the dominant [Mg/Fe] versus [Fe/H] trend; these stripes correspond to short bursts of star formation during which core-collapse supernovae enrich the surrounding medium with Mg (and Fe) on short time-scales. If observed, these features would substantiate the utility of elemental abundances in revealing the assembly and star-formation histories of dwarf galaxies. We explore the observability of these features for upcoming spectroscopic studies. Our results show that precise measurements of elemental abundance patterns can reveal critical events in the formation histories of low-mass galaxies. 

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5. On the Hunt for the Origins of the Orphan–Chenab Stream: Detailed Element Abundances with APOGEE and Gaia

   https://doi.org/10.3847/1538-4357/acb698  

   Hawkins, Keith; Price-Whelan, Adrian M.; Sheffield, Allyson A.; Subrahimovic, Aidan Z.; Beaton, Rachael L.; Belokurov, Vasily; Erkal, Denis; Koposov, Sergey E.; Lane, Richard R.; Laporte, Chervin F. P.; et al (May 2023, The Astrophysical Journal)

   Abstract Stellar streams in the Galactic halo are useful probes of the assembly of galaxies like the Milky Way. Many tidal stellar streams that have been found in recent years are accompanied by a known progenitor globular cluster or dwarf galaxy. However, the Orphan–Chenab (OC) stream is one case where a relatively narrow stream of stars has been found without a known progenitor. In an effort to find the parent of the OC stream, we use astrometry from the early third data release of ESA’s Gaia mission (Gaia EDR3) and radial velocity information from the Sloan Digital Sky Survey (SDSS)-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to find up to 13 stars that are likely members of the OC stream. We use the APOGEE survey to study the chemical nature (for up to 10 stars) of the OC stream in theα(O, Mg, Ca, Si, Ti, and S), odd-Z(Al, K, and V), Fe-peak (Fe, Ni, Mn, Co, and Cr), and neutron-capture (Ce) elemental groups. We find that the stars that make up the OC stream are not consistent with a monometallic population and have a median metallicity of −1.92 dex with a dispersion of 0.28 dex. Our results also indicate that the α elements are depleted compared to the known Milky Way populations and that its [Mg/Al] abundance ratio is not consistent with second-generation stars from globular clusters. The detailed chemical pattern of these stars, namely the [α/Fe]–[Fe/H] plane and the metallicity distribution, indicates that the OC stream progenitor is very likely to be a dwarf spheroidal galaxy with a mass of ∼10⁶M_⊙. 

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